Stop collars are used in downhole environments, for instance in the oil and gas industry, to mount around a tubular member such as a length of pipe, drill string or tubing string to engage and grip the exterior of the tubular member. Stop collars provide a stop shoulder on the tubular member to restrict axial travel along the tubular member of any further associated product—for example a centralizer—that is assembled onto the exterior of the tubular member.
As known to those skilled in the art, a stop collar, sometimes referred to as a stop ring or similar terminology, is commonly used to restrain the axial movement of products such as but not limited to centralizers that are assembled onto the tubular members (sometimes referred to as “tubulars”) of a well casing.
Centralizers are devices that engage over a tubular member, as above, and that have an external envelope intended to contact the bore to maintain that tubular member generally out of contact within—and ideally central within—the bore.
Stop collar design must cope with free fitment onto tubulars having poorly toleranced outer diameters. The reader is directed to American Petroleum Institute API 5CT which states that the tubular outer diameter tolerance is “nominal diameter+1%”. It may be seen that a most common tubular size of “nine and five-eights” (9⅝ inch, 24.47 cm) could be 9.625 inch to 9.721 inch (24.47 cm to 26.92 cm) outer diameter. Any design applied must take up this tolerance as pre-requisite to applying sufficient load to give the desired axial load restraint.
The many current stop collars or like devices used to resist axial loading rely on various methods of partially penetrating into the surface of the tubulars under action of locally applied axial loads. Two of the most common methods employed are toughened steel screws radially dispersed around the circumference of the stop collar, and hardened steel inserts wedged between the stop collar and the tubular surface.
Penetration of the surface of the tubulars creates significant marking which can lead to stress concentration and cause stress corrosion cracking when the tubular is placed in its operating environment. Where tubulars consist of an alloy containing for example chrome, commonly 13% or more, galvanic corrosion between the toughened steel screws and the chrome alloy surface exacerbates the tubular life failure rate.
Current arrangements are unable to resist axial loads of a magnitude similar to the load bearing capabilities of the associated components they are supposed to hold in position i.e. centralizers in either tension or compression. Increasing the number of radially disposed screws or wedges dramatically increases the stress corrosion potential. Users seek to balance between desired axial holding ability and the said increase in stress corrosion.
It is a further problem that assembly of the stop collar onto the tubular, in the field, is frequently delegated to unskilled labour. It is common practice to assemble, for example screws, with little regard to correct torques applied or to whether the threads are suitably lubricated. This latter point has an inbuilt hazard in that screws are frequently split, through incorrect torque applied, which will not be apparent to the personnel carrying out the assembly. The result possibly leads to even lesser axial holding ability as the tubular is traversed into its operating position. By default the screws employed must be small enough to fit with suitable clearance within the annulus formed between the tubular on which they are affixed and the wellbore or internal diameter of previously installed larger tubular, said screws commonly being 1.27 cm×1.27 cm (½″×½″) long socket set screws which have only a 0.635 cm (¼″) across flats hexagonal drive form. Hexagonal wrenches are small, have a very short life and the tendency is not to change for new hexagonal drives before rotational failure of the hexagonal drive corners, with resultant insufficient torque input to achieve desired axial holding forces.
The protrusion of screws or wedge devices beyond the outer diameter of the stop collar main body considerably restricts the use of traditional stop collars in a narrow annulus configuration existing between the tubular to which the stop collars are affixed and the wellbore or internal diameter of a previously installed larger tubular.
The aforementioned design practices of multiple part stop collar constructions may result in lost parts of the stop collar, or associated components, falling into the wellbore. This is considered as catastrophic in the industry.
Problems also occur with centralizers where the bore has an upper part of a generally smaller cross section than a lower part where the centralizer is needed to act. Clearly the centralizer must pass through the upper part without breakage, and without requiring too great an insertion force. The two constraints may of course be interrelated.
One such scenario is with so-called “under-reamed” bores. This occurs for example where wellbores are ‘opened out’ in a region lower than a previously installed tubular.
In one example, a drill bit is passed through the 21.68 cm (8.535″) internal diameter. of a previously installed 24.45 cm (9⅝″) tubular and then the bit is rotated out of concentric to create a 24.13 cm (9.5″) hole. So, a centralizer is required to fit the nominal size of 24.13 cm (9.5″) diameter so as to centralize a tubular in that bore, but also is required to pass through 21.58 cm (8.535″) diameter of the upper tubular.